3.3 meiosis Flashcards
n
haploid number of chromosomes; 23 in humans; sex cells
2n
diploid number of chromosomes; 46 in humans; somatic cells;
what does meiosis start and end with?
starts with 1 diploid parent cell, ends with 4 haploid daughter cells
bivalent
pair of homologous chromosomes
prophase 1 (4 parts)
- crossing over b/w non-sister chromatids, resulting in genetic variation
- chromosomes condense, nuclear membrane dissolves
- genetic recombination
- centrioles travel to poles
interphase
DNA is replicated to produce two genetically identical copies (sister chromatids held together at the centromere)
crossing over
Crossing over can happen between non-sister chromatids when the homologous pairs line up.
It results in the recombination of alleles and leads to genetic variation in gametes.
chiasmata
X-shaped points of attachment between two non-sister chromatids of a homologous pair
draw crossing over
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what happens in first meiotic division (meiosis 1)?
reduction division (diploid → haploid) in which homologous chromosomes are separated
what happens in meiosis 2?
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
metaphase 1
Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell. Random orientation
anaphase 1
Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell (sister chromatids stay together, but homologous pairs are pulled apart)
how does random orientation lead to genetic variation?
The homologous pairs do not always line up in the same way. So the different combinations always occur resulting in further variation.
telophase 1 (3 parts)
- chromosomes decondense, nuclear membrane reforms
- cleavage furrow, cytokinesis
- reduction division
draw meiosis 1
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draw meiosis 2
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draw both meiosis 1 and 2
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prophase 2 (3 parts)
- Nuclear membrane dissolves
- Chromosomes (with two sister chromatids connected by a centromere) condense.
- Centrioles travel to the poles.
*no crossing over occurs
metaphase 2 (2 parts)
- Microtubule spindle fibres connect to the centromeres.
- Chromosomes (two sister chromatids) line up along the equator
anaphase 2 (2 parts)
- Spindle fibres contract and separate the sister chromatids, centromere divides
- chromatids (now called chromosomes) move to opposite poles
telophase 2 (3 parts)
- Nuclear membrane reforms,
chromosomes decondense - Cytokinesis occurs
- 4 genetically unique nuclei created
purpose of meiosis
to produce sex cells for sexual reproduction
how does genetic variation happen in meiosis?
crossing over (prophase 1), or random orientation (metaphase 1 & 2)
why is genetic variation important?
Increased genetic variation results in a more resilient population. The population as a whole will be better able to withstand environmental changes, such as disease. Genetic variation is essential for successful change by evolution.
karyogram
a photograph of chromosomes in the nucleus. It is obtained by staining a cell undergoing cell-division (this division is halted).
how are human adult karyograms usually obtained?
from white blood cells that still divide in culture
how are karyograms obtained from unborn fetus’s?
obtained from interphase cells from the amniotic fluid (this is obtained by an amniotic synthesis or a chorionic villus sample).
karyotype
a property of a cell and describes its number of chromosomes and their appearance (banding, centromere position, size, and shape)) in an individual/species.
what can a karyogram be used for?
determine sex, chromosomal disorders such as Down syndrome (trisomy 21)
non-disjunction
the failure of homologous chromosomes or sister chromatids to separate properly during meiosis (anaphase I or anaphase II).
what can non-disjunction lead to?
Down syndrome and other chromosomal disorders
3 methods to obtain cells for karyotype analysis
- nuchal translucency scan
- amniotic centesis
- chorionic villus sample
nuchal translucency scan
- performed at week 11-13
- Non-invasive. Uses ultrasound to view the nuchal fluid behind the neck of the fetus. Large amounts correlate with Down Syndrome.
- 0% chance of miscarriage
amniotic centesis
- performed at week 14-16
- A needle is inserted through the mother’s abdominal wall and punctures the amniotic sac (ultrasound guided). Amniotic fluid is collected
- 0.5% chance of miscarriage
chorionic villus sample
- performed at week 10-12
- Placental tissue is obtained either through the vagina/cervix or the abdominal wall.
- 1-2% chance of miscarriage
